An evolution trend from a homogeneous network to a heterogeneous network is presented during a network evolution process of a Long Term Evolution (LTE) system and a Long Term Evolution Advanced (LTE-A) system. In a heterogeneous network architecture, more small cell (SC) devices are increased within coverage of a macro base station (Macro evolved Node B, Macro eNodeB), to further increase a network coverage capacity. A small cell device refers to a low-power radio access node. Compared with a macro base station, the small cell device has lower power, a smaller coverage area, and a smaller size. For example, the small cell device includes a home eNodeB (Home eNodeB), a micro base station (Pico eNodeB), and a remote radio head (Remote Radio Head, RRH).
With increasing requirements for the network coverage capacity, small cell densification is a main solution to the increasing requirements for the network coverage capacity. However, there are many actual difficulties in densely deploying small cell devices. The following problem is caused during data transmission over a backhaul (backhaul) between numerous small cell devices because of synchronization between small cells. The backhaul generally refers to a data transmission path between a small cell device and a core network (Core Network). A small cell device accessed by user equipment (User Equipment, UE) is referred to as an AP. The AP transmits, to the core network by means of forwarding of multiple small cell devices, data sent by the UE. Data transmission paths between small cell devices on a data transmission path from the AP to the core network are referred to as backhauls. In a network of densely deployed small cell devices, user equipment (User Equipment, UE) may be frequently handed over between multiple small cell devices. Therefore, a backhaul path of a small cell device needs to be flexibly changed. In a flexible backhaul technical solution, during data transmission between small cell devices, a transmission path and a transmission manner may change. When a transmission manner of a small cell device is changed, for example, when the small cell device switches from a state of sending an uplink subframe to a state of sending a downlink subframe, another small cell device that receives data sent by the small cell device needs to first spend a specific period of time synchronizing with the small cell device that sends the data. In this period of time, data transmission cannot be performed between the small cell devices.
Generally, there are multiple manners to implement a backhaul, such as a fiber, a microwave, and a relay (Relay). However, if a wired backhaul manner, such as the fiber, is adopted, a new route needs to be set, and this causes relatively high costs. The network of densely deployed small cell devices is generally in a mature city region, and it is unlikely to carry out a setting project of a new route. Therefore, a wireless backhaul manner is a main development direction for a manner of densely deploying small cell devices.
In a relay (Relay) technology of the wireless backhaul manner, when transmitting an uplink subframe, a small cell device periodically retains one downlink subframe and one special subframe, and transmits a synchronization signal on the special subframe, so that a small cell device that detects the synchronization signal can keep synchronous with the small cell device that transmits the uplink subframe. However, in a process of transmitting the uplink subframe, the small cell device needs to always periodically retain a configuration of one downlink subframe and one special subframe. Therefore, configuration flexibility of a transmission manner of the downlink subframe and the special subframe is sacrificed, and valuable transmission resources are wasted.